Practical dechlorination of polyvinyl chloride wastes in NaOH/ethylene glycol using an up-scale ball mill reactor and validation by discrete element method simulations

Abstract

To avoid the formation of undesired Cl compounds during polyvinyl chloride (PVC) wastes treatment and facilitate the recycling of valuable NaCl and dechlorinated hydrocarbons as feedstocks, advanced dechlorination (de-Cl) process should be developed. Here, an up-scale ball mill reactor was established for the de-Cl of real PVC wastes, including sealing strips from waste refrigerators and crushed cable coverings from waste cables. The effects of NaOH on de-Cl were validated with lab-scale studies and the influences of mechanical conditions were innovatively investigated. A maximum de-Cl degree of 99% was obtained with 1 M NaOH in ethylene glycol for sealing strips, whereas a maximum de-Cl degree of 92% was obtained with Φ1.27 cm stainless steel balls at a moderate rotation speed for cable coverings. The remaining Cl content in the sample residues was small and decreased with decreasing residue size, resulting in minimum contents of 0.49% and 0.61% for sealing strips and cable coverings, respectively. The de-Cl behavior was consistent with a shrinking-core model and the meaning of kinetic parameters was illustrated. The ball milling process was simulated by discrete element method (DEM). A positive correlation was observed between the apparent rate constant of the experimental de-Cl process and the specific impact energy calculated using DEM simulations. The combined experimental and simulation approach suggested that the surface of PVC is first dechlorinated and then crushed into fine particles by ball milling to expose the inner unreacted surface. For industrial application, the balance of chemical and mechanical conditions should be optimized.